The manufacture of optical fiber typically uses one of two fundamental approaches. Both use rotating lathes, and accumulate pure glass material on a rotating preform by chemical vapor deposition or a modification thereof. The earliest technique deposited material on the outside of a rotating preform, and the preform usually started as a hollow tube with a slowly increasing diameter as the vapor deposited glass material accumulated on the outside of the solid tube. A significant advance in this technology occurred with the introduction of the so-called Modified Chemical Vapor Deposition (MCVD) process of MacChesney et al in which the glass forming precursers were introduced into a rotating hollow tube and the glass material was deposited on the inside wall of the hollow tube. In this way exceptionally pure material could be produced at the critical core region. It also allowed better control over the reaction environment.
The MCVD technique has evolved to a highly sophisticated manufacturing technique and is widely used in commercial practice today.
However, a basic technological problem inherent with the use of a hollow tube, a problem that has persisted since the discovery of the MCVD technique, is that of ensuring the circularity of the tube throughout the deposition process. It is an inherent thermodynamic condition of the process that the temperatures used during consolidation and collapse of the glass tube exceed the softening temperature of the initial glass tube so that throughout the process the tube itself is vulnerable to deformation. Typically such deformation results in small changes in the circular cross section of the tube producing tube ovality, and the tube is most susceptible to such changes during the tube collapse operation.
It has also been a challenge to provide tubes initially with a consistent circular profile along the full length of the tube. A successful technique that is capable of eliminating departures in tube circularity during the collapse process could also be effective in eliminating ovality in starting tubes by a predeposition treatment in which the torch is passed at elevated temperatures down the lathe to adjust the circularity of the starting tube.
To attempt to avoid ovality problems developing during tube collapse it has been customary in the technology to collapse the tube slowly, using multiple passes of the torch. The objective is to shrink the diameter in small increments so that the surface tension of the glass, which tends to preserve circularity, can be close to equilibrium and can offset other forces, e.g. gravity, that tend to produce ovality. However, in current manufacture the collapse process is undesirably long, typically consuming nearly half the manufacturing time. Thus an important objective in the MCVD art is to reduce tube collapse time. Techniques that allow better control over the ovality problem also allow more aggressive tube collapse schedules, which in turn substantially reduce the costs of preform manufacture.
A known prior art technique for controlling or eliminating ovality problems during the MCVD process is to maintain a positive pressure of an inert gas, e.g. nitrogen or argon, in the tube especially during collapse of the tube. If the initial tube is circular a uniform hydrostatic pressure inside the tube theoretically will equalize the surface tension of the collapsing glass both along the length of the tube and around the circumference of the tube to maintain uniform collapsing forces throughout the tube. While this approach has been successful in addressing the ovality problem, the use of internal pressure in the tube actually reduces the collapse rate. Techniques for reducing the duration of the tube collapse step while preventing tube ovality continue is a major goal of MCVD process designers.